Elemental phosphorus generally contains parts per million quantities of metals such as iron, arsenic, chromium, zinc, and antimony. The extent to which these metals are present in the phosphorus depends on the source of the phosphate ore that the phosphorus is extracted from and, to some extent, on the overall process used to produce and refine the elemental phosphorus. Of these metals, antimony has been most difficult to remove on an economical basis. Since a high antimony content can make the phosphorus unacceptable for use in certain food and electronics applications, it is important to develop a more economic method of removal.
This problem is perpetuated when elemental phosphorus is converted into phosphoric acid because the antimony in the phosphorus carries over into the phosphoric acid. Thus, in high value phosphoric acid products such as food grade acid and semiconductor grade acid, antimony can be a problem.
The problem is compounded by the fact that elemental phosphorus is not easy to work with. Because it spontaneously combusts when it is exposed to air, it is typically kept under a water blanket to prevent air contact. Because elemental phosphorus freezes at 44.degree. C., it is typically kept at a higher temperature to keep the phosphorus fluid, so that it can be readily worked with, stored, and transported. To meet both these objectives, it is typically kept at a temperature above its melting temperature of 44.degree. C. to facilitate handling, and below the boiling temperature of water to preserve the water blanket. Thus, elemental phosphorus is usually handled as a two phase system, consisting of a phosphorus layer that is covered by a layer of water.
Hydrogen peroxide has been shown to remove iron in an elemental phosphorus system that consists of three phases: a water phase, a phosphorus phase and an organic phase that consists of an organo halide solvent, such as carbon tetrachloride or trichloroethylene. In that somewhat complicated system, the water, phosphorus and organic solvent were mixed and the phosphorus was dispersed in a larger volume of organo halide solvent to facilitate reaction between the hydrogen peroxide and the iron contained in the phosphorus. The art has been silent as to the use of that process in systems containing antimony or in simple phosphorus and water systems.